Jeff McLane of the Philadelphia Inquirer reported Monday that Mathis "will be out for an extended period" after an MRI revealed a sprained MCL in his right knee, according to an NFL source.
Mathis will get a second opinion on Wednesday. It's possible the Eagles could opt to place the guard on short-term injured reserve, a move that would sideline him for at least eight weeks.
Mathis isn't the only issue on Philadelphia's line right now. Right tackle Allen Barbre suffered a high-ankle sprain and is expected to miss several weeks, a source told McClane. Barbre had been filling in for the suspended Lane Johnson. Andrew Gardner and Dennis Kelly are expected to slide into starting roles in Week 2.